Chapter 13 

 ANGLE-OF-ARRIVAL EXPERIMENTS 



13 1 ANGLE-OF-ARRIVAL MEASUREMENTS 

 IN THE X BAND^ 



THE PUKPOSE of this work was to observe the varia- 

 tion in angle of arrival of waves in the X band. No 

 simultaneous air sounding data were taken although 

 general weather observations were made. 



The method of measuring the angle of arrival makes 

 use of a very sharp-beamed antenna (Figure 1) 

 mounted so that it may be mechanically tilted back 



6 INCHES WIDE (15° HORIZONTAL BEAM) 



T 



20 FEET 



i 



ANTENNA SWINGS 

 ±.T5° 



Figure 1. .Sliarp-beamed antenna used for measuring 

 the angle of arrival. 



and forth al)out its center thus sweeping the beam of 

 the antenna through an arc which may be set to include 

 the expected angle of arrival of the incoming signal. 

 The sharp-beamed antenna has been used to measure 

 the angle of arrival of waves from a distant trans- 

 mitter over an optical path where both a direct wave 

 and a water-reflected wave are present. If the output 



A ONLY IRECORO 1) 



o \ UP / \down / \ up / \down / \ up / \ 



^ Vvvvy V \7\ 



klO SEC^ 



A+B (RECORD 2) 



Figure 2. Variation in signal intensity during scan. 

 A, direct ray only. B, direct and reflected rays super- 

 posed. 



of the receiving antenna is fed to a receiver and this 

 receiver is fitted with a recording type output meter, 

 records of the type shown in Figure 3 will be obtained 

 as the antenna scans. Eecord 1 will be obtained if only 

 a single, direct wave is arriving at an angle correspond- 



"By W. M. Sharpless, Bell Telephone Laboratories. 



ing to the mid-point of the antenna swing. The dis- 

 tances between peaks of maximum amplitude along 

 the record will then be equal. A shift in the angle of 

 arrival of the wave would appear on the record as a 

 change in the spacing between the peaks. If two 

 separate waves, direct and reflected, are arriving 

 simultaneously, the record will aj^pear something 

 like record 2. 



The actual antenna used for the measurement is a 

 section of a j)arabolic cylinder arranged so that its 

 beam at the center of swing is pointed directly at the 

 transmitter. This is also the angle at which waves 

 arrive on a normal day. A normal day has been taken 

 as one when the angle of arrival is the same (within 

 the accuracy of the measurements) as that calculated 

 from actual earth geometry and when free space field 

 is received from the direct wave. 



The lahysical jaosition of the antenna may be held to 

 approximately 1/100 degree by the use of a plum-bob 

 line dropped from the top of the 20-ft antenna to the 

 base. Possible errors in reading the records, however, 

 limit the expected relative accuracy to about 1/60 

 degree. Slight errors in the actual building of the 

 antennas and in the locating of the feed limit the 

 final accuracy to what is believed to be 1/25 degree. 



The horizontal angle of arrival is measured with a 

 duplicate antenna turned 90 degrees from the vertical 

 with its flat side toward the ground. The accuracy of 

 measurement is the same as in the vertical plane case. 



The entire equipment, including the two scanning 

 antennas, other reference antennas, the receiving 

 equipment, and the receiver building are located on 

 a rotatable platform which is 25 ft in diameter. This 

 equipment, located on top of Beer's Hill, New Jersey, 

 may thus be pointed toward any of several transmitters 

 and comparisons made of the angle of arrival from 

 each transmitter. 



Observations during the summer of 1944 have been 

 made on two optical paths shown in Figure 3 : (a) A 

 2-4. 1-mile path partly over land and partly over water 

 between New York City and Beer's Hill, New Jersey. 

 The normal reflecting point for the reflected ray on 

 this path is in the salt water of Baritan Bay; (b) a 

 12.6-mile path between Beer's Hill and Deal, New 

 Jersey. This path is all over gently rolling land. The 



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